1. Field of the Invention
The present invention relates to an optical device comprising a constitutive element of glass, and in particular, to a technique of improving the weather resistance of the optical device.
2. Related Art
Glass that contains an alkaline metal undergoes a phenomenon that the alkaline metal deposits on the surface thereof owing to the diffusive movement of the alkaline metal ions. The deposited alkaline metal may form a salt and adhere to the glass surface, or an alkaline metal component may dissolve out on the glass surface and the structure of the glass surface may change. This phenomenon is more remarkable in a high-temperature high-humidity environment (for example, in an environment at a temperature of 80° C. and a humidity of 90% RH), and this maybe a factor of worsening the weather resistance of glass in optical applications. The following three techniques are mainly known for preventing the phenomenon and for improving the weather resistance of glass:    (1) Heat treatment: This is surface modification of glass, comprising heating glass at 150° C. or higher so as to previously release alkaline and other monovalent ions (e.g., see JP-A 58-2243).    (2) Ion-exchange treatment: This is surface modification of glass, comprising dipping glass in a metal ion-containing molten salt to thereby exchange the readily-releasable metal ion in the glass with a metal ion not readily releasable even at high temperatures, and this is for imparting metal ion release resistance to glass (e.g., see JP-A 58-2243 and JP-A 60-260445).    (3) Chemical treatment: This is surface modification of glass, comprising contacting glass with warm water or with an aqueous acid solution to thereby release the alkaline metal ion that exists in a vicinity of the glass surface (e.g., see JP-A 11-171599).
The weather resistance test of glass lenses made by the present inventors has revealed that divalent alkaline earth metal ions may also deposit in a high-temperature high-humidity environment. However, divalent alkaline earth metal ions are difficult to release by heat treatment.
In ion-exchange treatment, glass members may be cracked, or may be deformed or deteriorated. Chemical treatment is also problematic in that glass members may be cracked through it.